1. Field of the Invention
The present invention relates to sonar mine detectors, namely sonars that enable the detection of underwater objects, that have sent back sonar echoes, through display of these echoes, and that make it possible, notably, to find out whether these objects are mines floating in the water, particularly moored mines, or mines laid on the seabed.
2. Description of the Prior Art
As is known, moored mines are mines floating within the water, in being hooked by a cable, called a mooring cable, to a mooring buoy, called a sinker, lying at the bottom of the seabed.
Mine-sweepers are commonly fitted with an active sonar which works at relatively high frequencies of several tens of kHz, to have an angular resolution compatible with the size of the mines.
The sonar is mounted either on the mine-sweeper or on a "fish" (or unmanned submarine type device) which is towed by the mine-sweeper or is self-propelled.
This sonar has an emitting antenna which diffuses sound in the marine environment to obtain uniform sound diffusion in a volume extending, for example, over 30.degree. in bearing and 80.degree. in dip. The sonar also has a reception antenna enabling the formation of directional channels, in bearing, to obtain the angular resolution desired according to the bearing. In thus forming, for example, as shown in FIG. 1, 100 channels such as 13, with an angle width of 0.3.degree. , it is possible to cover an sound-diffused angle sector equal to 30.degree..
FIG. 1 shows the volume sound-diffused from a sonar mounted on a "fish". It corresponds to an observation zone 12 demarcated, in bearing, by the angles .alpha..sub.1 and .alpha..sub.2 and, in distance, by the distances d.sub.M and d.sub.m. Of these distances, d.sub.M corresponds to the maximum range beyond which the echo becomes too weak and d.sub.m corresponds to a distance close to the height of water between the "fish" and the seabed. The seabed is sound-diffused flatly.
The FIG. 2a shows the zone 12 explored by the sonar as can be seen on the display screen of this sonar in mode B (bearing-distance). Each channel, such as 13, corresponds to an angle between and .alpha.;, between .alpha..sub.1 and .alpha..sub.2, and is represented by a strip shown enlarged in FIG. 2 for the requirements of the drawing. Every echo detected within the channel 13 will be represented, on the corresponding strip on the display screen, by a bright dot. This bright dot will appear on a less luminous background at a distance d, irrespectively of its height over the seabed. It follows that it cannot be ascertained, as shown in FIG. 2b, whether the echo corresponds to a mine 21 laid on the seabed or to a moored mine 22.
To make it possible to distinguish between echos corresponding to objects laid on the seabed and those corresponding to objects floating in the water, the invention proposes to modify the dip emission diagram of the sonar to obtain two main lobes separated by a zero value located in the field of observation of the sonar. A dark zone is obtained on the display screen, corresponding to the oblique distance from the zero value. When an object is located on the seabed at this distance, the echo corresponding to this object disappears in the dark zone, whereas when there is an object floating in the water, at the same oblique distance, the echo remains and its contrast increases, with respect to the dark zone into which it passes, enabling the operator to classify the object.